Aircraft: SMALL AIRCRAFT; LOW WING; 1 ENG; FIXED GEAR
Phase of flight: Final Approach
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy

Aircraft X was descending southbound over the top from NKX airspace north of the airport. Aircraft Y requested a left 270 turn northbound which was denied to avoid conflict with Aircraft X. Aircraft Y was issued a right downwind departure and switched to SCT. He continued his climb and began converging on Aircraft X. I called SCT and issued a traffic alert for them. They had already issued traffic as well. The pilot of Aircraft X reported the Aircraft Y in sight; but said it was close; so a MOR was filed. What led to the event was the lack of defined procedures between us and SCT when NKX is closed. It is a grey area with no strict definition in the LOA. Traffic comes to us on random routes and sometimes with or without restrictions. Normally; since we have almost no way of knowing (even though scratchpad entries are supposed to be made); I almost always issue 'comply with previous restrictions' to help cover myself. On the same note; the issue is similar in the transitions from NKX to us; because both northbound and southbound aircraft are issued the same altitudes which have resulted in conflicts on some occasions. The SCT-MYF LOA needs to define clear procedures to follow for coordination of aircraft through the Class B northbound and southbound. As it is; we have nothing in writing; and there is no standard; except for the inefficient 'Class B' clearance which we find confuses some SCT controllers. What we need are clearly defined routes and altitudes for VFR aircraft to help prevent this sort of situation. There is nothing in place; it is just seat-of-the-pants controlling. The same issue needs to be addressed with the NKX-MYF LOA to define routes and/or altitudes to be flown in order to avoid the pre-programmed conflict. I was approaching MYF from the North having just been handed off by SOCAL to Montgomery Tower. They advised me to continue approach as ordered by SOCAL; cross the airport at midfield at 2;000 feet MSL or greater then enter left downwind for left traffic for Runway 28L. I acknowledged and then noticed there was converging traffic in the right downwind that appeared to be climbing. I notified the Tower and they said the traffic was under the direction of ATC. I continued to monitor the traffic and saw that it was; in fact climbing. As it got close; I initiated a small climb to create a little extra margin of space. When the traffic disappeared as it went under me; I glanced at my MFD and noted that the traffic had passed under me at -200 feet. I contacted the Tower and commented that the traffic had passed about 200 feet below me (thinking 'that's a little close.') When I landed; the Tower gave me a phone number. When I called it; I spoke to a man who identified himself as the Tower Manager. He took my information and said he would mail a copy of the report to me when it was completed. Subsequently; when I was on final and cleared to land; the Tower released traffic on my intended runway and I was forced to go around. I executed a left 180 re-entering the downwind leg and the Tower said they would call my base. The rest of the approach and landing was without incident. The Tower was training new controllers as they often do and I never felt there was a dire emergency. Just new people learning their job with a couple of learning errors. MYF Controller described a conflict event involving an arrival VFR aircraft; the reporter claiming the procedures with SCT need to be revised to more clearly define routes/procedures and coordination to be used in these types of situations.